Due Date Calculator
Estimate pregnancy due date
About the Due Date Calculator
For expectant parents, the estimated due date is the anchor everything else is planned around. This calculator applies Naegele's rule — the standard method clinicians use — adding 280 days to the first day of your last menstrual period to estimate when your baby is due.
It gives a quick, recognized estimate for early planning, appointments, and counting down the weeks. While an ultrasound and your healthcare provider give the most accurate dating, the last-period method this tool uses is the widely accepted starting point and matches how due dates are first calculated. Keep in mind that only about one in twenty babies arrives on the estimated date itself — the figure marks the middle of a window, not a deadline.
How to Use the Due Date Calculator
- Select the first day of your last menstrual period.
- Read your estimated due date, calculated as 280 days later.
- Use it as a planning anchor, and confirm timing with your healthcare provider.
Why Use ToolForge’s Due Date Calculator
- It uses Naegele's rule (last period plus 280 days), the standard method clinicians use to estimate due dates.
- It needs just one date and returns the estimate immediately, with no forms or sign-up.
- The result is a clear calendar date you can plan around.
- Your information is processed locally and never leaves your browser.
Frequently Asked Questions
How is a due date calculated?
The common method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. This calculator applies that rule directly. It assumes a regular 28-day cycle, which is why a provider may adjust the date.
How accurate is the estimated due date?
It is an estimate — only about 1 in 20 babies arrive exactly on their due date, and most are born within a couple of weeks either side. An early ultrasound often refines the date, especially if your cycle is irregular.
What if I don't know my last period date or have irregular cycles?
The last-period method is less reliable with irregular cycles or an uncertain date. In those cases, dating is usually based on ultrasound measurements, so check with your healthcare provider for a more accurate estimate.
Can the due date change later in pregnancy?
Yes. Providers often refine the date after an early ultrasound, which measures the baby directly and can be more accurate than the last-period estimate, especially with irregular cycles. The figure here is a starting point, not a fixed appointment.
